This invention pertains to an improvement in a process for curing ethylenically unsaturated vehicles used as binders for pigments and fillers in surface coatings as well as clear non-pigmented paint films.
It is well known that conventional convection ovens or infrared ovens or other heat sources may be used to heat cure or polymerize various polymeric binders used in surface protective coatings wherein curing is promoted by a catalyst provided in the coating system. More recently, however, a broad spectrum of ultraviolet wave energy curing has been suggested for polymerizing binders wherein suitable ultraviolet sensitizers are incorporated in the coating system for initiating polymerization of the binder. The UV sources ordinarily produce wavelengths in the UV spectrum that are transmittable through a quartz and generally provide useful wavelengths between about 1600 Angstroms and about 4000 Angstroms. Typical UV emittors include various electric arc lamps, the plasma arc torch described in U.S. Pat. No. 3,364,387. The subject matter of the foregoing references are incorporated herein by reference.
It now has been found that UV curing of ethylenically unsaturated binders can be substantially improved by incorporating a UV sensitizer into the polymer backbone wherein the polymer structure is an ordered polymer design such as a block or star polymer. The UV photosensitizer is pendantly attached to a polymer chain and particularly spaced in the polymer from the ethylenic unsaturation by providing the UV sensitizer as being pendantly attached to a second polymer chain. The polymers are particularly formed in a step-wise process to space the UV sensitizer in the polymer from the ethylenic double bond whereupon curing with ultraviolet radiation until the paint coating is hard and tackfree provides excellent adhesion of the coating to the substrate.
Accordingly, a primary objective of this invention is to incorporate a UV sensitizer into the polymer backbone and specifically space the sensitizer from the ethylenic unsaturation by the intervening polymer structure.
A further advantage is to provide a substantially improved UV curing process for room temperature curing of polymers with UV sources wherein loss of UV sensitizer is avoided by incorporating the UV sensitizer into the polymer backbone. Other advantages include suppressing volatilization of solvents; improving the color and avoiding degradation of the protective film; avoiding degradation of substrates such as plastic, paper, or fabric. Room temperature UV and laser curing is rapidly promoted and highly efficient even though the film is fairly thick, high pigmented and/or heavily filled with inerts.
These and other advantages of this invention will become more apparent from the detailed description of the invention.